Since the development of the incandescent lamp, more efficient illumination devices have constantly been sought, along with qualities of improved light output spectrum, durability and useful lifetime. While the incandescent lamp is highly versatile in that light output flux can be varied with simple control of the power applied thereto, the efficiency of conversion of input power to light output is notoriously poor and the spectrum of light output is much different from natural sunlight and varies with input power. Incandescent lamps also have a relatively short useable lifetime and are subject to being broken or rendered inoperative from even relatively small levels of impact or vibration.
Well-known fluorescent lamps provide much greater efficiency and useful lifetime but are generally of larger size and substantially fixed light output flux of generally poor spectral quality and may even be more delicate and subject to damage than incandescent lamps with the potential for release of toxic substances. Further, since fluorescent lamps are operated using a discharge in a gas, special power supplies, often of significant weight and/or bulk, are required.
In recent years, light emitting diodes (LEDs) have become a good candidate for general illumination as well as for use in selective displays and signaling. LEDs are highly efficient, of very long useful service life, extremely rugged and the spectral output for a given light output flux can be readily established by choice of the materials and/or construction of the LED. Moreover, individual LEDs and small arrays of LEDs can be driven at relatively low voltages from simple power sources such as batteries or very simple power converters that can thus be of very small size, particularly where the spectral content of the output light is not of particular importance. Further, small arrays can be configured to have good uniformity of light output flux over at least a small area; making LEDs a particularly good candidate for illumination lamps and backlighting of passive display panels (e.g. liquid crystal screens). LEDs also exhibit a faster response time when energized or de-energized and do not present any significant ecological hazard.
However, to meet more general illumination requirements such as indoor or outdoor lighting of spaces of significant size and backlighting of large scale display panels, very large arrays of LEDs must be used since the light output of an individual LED is relatively small. Further, for good stability of light spectral content and to preserve the long service life of LEDs, they must be driven with a substantially constant current. Such larger arrays of LEDs are thus usually constituted by a plurality of long, serially-connected strings of LEDs where the number of serially connected LEDs may range from several dozen to several hundred.
Since, in operation, a given LED will exhibit a forward voltage drop when a current sufficient to cause illumination is passed through it, such strings of LEDs generally require a high DC voltage equal to the sum of all the forward voltage drops in a serially connected string of LEDs. Further, since one failure mode of an LED is to fail in a shorted condition (where no significant forward voltage drop will occur even while current is passed therethrough) the voltage which must be applied to each serially connected string of LEDs may differ markedly between the serially connected strings which comprise a given array. Thus, power supplies for larger LED arrays suitable for illumination have generally required each serially connected string of LEDs to be driven by a separate constant current power supply (e.g. a constant current supply being provided for each string of LEDs). It can be readily appreciated that the need to drive a potentially large plurality of strings of LEDs with a separate and separately controlled power supply channel for each LED string with coordination of current levels between separate constant current sources can cause an LED driver to require substantial complexity, cost and bulk, even when power factor correction and front-end DC-to-DC converter can be provided in common for all channels. Further, the use of three stages in the power supply tends to compromise the efficiency of the combination of the LED array and driver even though the energy conversion efficiency of the LEDs may be very high. Additionally, to emulate incandescent lamps in regard to the capability of varying output light flux, it is necessary to control the current and voltage applied to LED strings used for illumination. Such a requirement for variable current and voltage, particularly a current and voltage which must be varied over a wide range, can also compromise efficiency, particularly in resonant converters, as well as being complicated by the number of series-connected LEDs in each string and the number of strings which are commonly driven.